This invention relates to a fibre optic grating sensor, particularly but not exclusively a fibre optic grating strain or temperature sensor.
It Is known that fibre Bragg gratings may be used as a tool for measuring, for example, temperature and strain. The grating is formed in the fibre to reflect light at a wavelength which is determined by the physical characteristics of the grating. A change in the temperature and/or strain applied to a fibre Bragg grating changes the period of the grating and hence the wavelength of the light reflected by the grating. Arrays of fibre Bragg gratings, in which the gratings are spatially separated along a length of fibre and the spectral profiles of the gratings are separated in wavelength have been used to measure strain and temperature gradients by illuminating the array with broadband light and measuring the wavelength of the reflected light. A measurement of the wavelength shift (if any) provided by each grating provides information about the strain or temperature at that grating. Using a grating array to make a quasi-distributed strain or temperature measurement along a short length (about 5 cm or less) requires the fabrication of grating arrays comprising gratings of very short spatial length (generally less than 4 mm). For short length arrays, the spatial separation between adjacent gratings often becomes comparable to the spatial length of the gratings. In order to increase the number of gratings within the array the length of the gratings must be decreased, which rapidly increases the spectral bandwidth of the grating and hence reduces the spectral resolution of the sensor.
It Is an object of the present invention to provide an improved fibre optic grating sensor.
According to one aspect of the present invention there is provided a fibre optic grating sensor comprising an optical fibre having a grating portion along which the refractive index of the fibre varies periodically and at least substantially continuously to form an at least substantially continuous trating structure, the periodic variation having an amplitude envelope which includes at least one region in which the amplitude of the envelope is substantially reduced, the said variation giving the grating portion a spectral profile within which there is at least one pass band.
Preferably, the amplitude envelope includes at least one region in which the amplitude of the envelope is substantially nulled. The amplitude envelope preferably includes a plurality of regions in which the amplitude of the envelope is substantially nulled. Each null region desirably gives rise to a corresponding pass band.
Preferably, the periodic variation in the or each region includes a section in which the phase of the periodic variation substantially reverses, the or each reduction in the amplitude of the refractive index envelope and the or each corresponding phase reversal acting to give rise to a corresponding pass band within the spectral profile of the grating portion.
Adjacent null regions and hence adjacent sensor elements, are desirably spatially separated. Preferably each sensor element is independently actuable and hence the wavelengths of the corresponding pass bands are independently variable.
The grating portion preferably comprises two substantially superimposed fibre Bragg gratings. The amplitudes of the refractive index profiles of the two gratings preferably add together to form the amplitude envelope and the or each phase change.
The fibre Bragg gratings are preferably chirped fibre Bragg gratings. Desirably, the two chirped gratings have substantially the same rate of chirp and substantially the same spectral bandwidth, the first chirped grating having a different central wavelength to the second chirped grating. Alternatively, the first chirped grating has a different rate of chirp to the second chirped grating, and the two chirped gratings have substantially the same central wavelength and bandwidth.
Alternatively, the fibre Bragg gratings may be linear fibre Bragg gratings. The two linear gratings preferably have substantially the same spectral bandwidth. Desirably the first linear grating has a different central wavelength to the second linear grating.
The grating portion may alternatively comprise one fibre Bragg grating having a plurality of regions within which the refractive index profile of the grating is substantially reduced or nulled.
The fibre Bragg gratings are preferably fabricated using a known two-beam interference holographic fabrication method. Alternatively, the fibre Bragg gratings may be fabricated using a known phase-mask fabrication method.
The null regions in the single fibre Bragg grating are desirably formed in a fibre Bragg grating fabricated using the two-beam interference holographic fabrication method by providing an amplitude mask generally in front of the fibre, generally in the beam paths, during fabrication. Alternatively, the null regions in the single fibre Bragg grating may be formed in a fibre Bragg grating fabricated using the two-beam interference holographic fabrication method or the phase-mask fabrication method by subsequently further exposing regions of the grating. The single fibre Bragg grating may be a linear grating or a chirped grating.
The grating portion may further alternatively comprise a single grating structure fabricated using the phase-mask fabrication method. The desired grating structure is preferably represented on a phase-mask and subsequently inscribed into the fibre through the phase-mask. Alternatively, the grating structure may be inscribed in the fibre through a phase-mask, the fibre undergoing oscillating motion, along its longitudinal direction, relative to the phase-mask to thereby control the refractive index profile within the grating structure. The phase-mask may alternatively or additionally undergo oscillating motion relative to the fibre.
According to a further aspect of the present invention there is provided a fibre optic grating sensor comprising optical fibre having a grating portion along which the refractive index of the fibre varies periodically and at least substantially continuously to form an at least substantially continuous trating structure, the periodic variation including at least one section in which the phase of the periodic variation substantially reverses, the said variation giving the grating portion a spectral profile within which there is at least one pass band.
The grating portion preferably includes a plurality of sections in which the phase of the periodic variation substantially reverses. The or each phase reversal section desirably gives rise to a corresponding pass band. Adjacent phase reversal sections, and hence adjacent sensors, are desirably spatially separated. Preferably each sensor is independently actuable and hence the wavelengths of the corresponding pass bands are independently variable.
Preferably, the period of the periodic variation changes along at least part of the grating portion, and most preferably changes along substantially the full length of the grating portion.
The grating portion preferably comprises two substantially overlapping chirped fibre Bragg gratings, the first chirped grating being spatially shifted relative to the second chirped grating by an integer plus a fraction of the period of the first grating. The first chirped grating is preferably spatially shifted relative to the second chirped grating by an integer plus one half of the period of the first grating.
Desirably, the two chirped gratings have substantially the same rate of chirp and substantially the same spectral bandwidth, the first chirped grating having a different central wavelength to the second chirped grating. Alternatively, the first chirped grating has a different rate of chirp to the second chirped grating, and the two chirped gratings have substantially the same central wavelength and bandwidth.
The fibre Bragg gratings are preferably fabricated using a known two beam interference holographic fabrication method. Alternatively, the fibre Bragg gratings may be fabricated using a known phase-mask fabrication method.
Alternatively, the grating portion may comprise one chirped fibre Bragg grating having a plurality of sections in which the phase of the periodic variation substantially reverses.
The single chirped fibre Bragg grating is preferably fabricated using a known phase-mask fabrication technique. Preferably, a first part of the chirped grating having a first spectral bandwidth is represented on a phase-mask and subsequently inscribed into the fibre. A second part of the chirped grating having a second spectral bandwidth is preferably inscribed into the fibre substantially spatially and spectrally adjacent the first part, the first and second parts together forming the chirped grating. Preferably, the chirped grating comprises a plurality of such parts arranged substantially spatially and spectrally sequentially adjacent one another.
Preferably, the fibre is under a first strain during inscription of the first part. The fibre is preferably under a second strain during subscription of the second part, the first and second strains being different.
The chirped grating may alternatively be fabricated using a single phase-mask, the desired structure of the chirped grating being represented on the phase-mask and subsequently inscribed into the fibre through the phase-mask.
The chirped grating may further alternatively be fabricated using a known continuous writing technique.
The optical fibre is preferably photosensitive enhanced optical fibre. The photosensitive enhanced optical fibre may be germania doped optical fibre, or boron-germania co-doped optical fibre. The germania doped fibre may be hydrogen loaded to further increase the photosensitivity of the fibre. The boron-germania co-doped fibre may be hydrogen loaded to further increase the photosensitivity of the fibre. The photosensitive enhanced optical fibre may alternatively be hydrogen loaded standard optical fibre. The hydrogen loaded fibre may be annealed following fabrication of the grating structure to substantially remove any residual hydrogen from the fibre.
The grating structure may further alternatively comprise a side-etched surface-relief grating structure, preferably fabricated in standard optical fibre.
The fibre grating sensor may comprise a plurality of grating portions.